Prior geochemical studies have reported that produced waters from shale gas and tight oil are a mixture of injected fluids and formation waters, with the latter dominating after the initial return of injected fluids. The assumption that later-stage produced water is largely representative of formation waters forms the basis of the current understanding of the source and behavior of deep fluids in shales and mudstones. Here, for the first time, we demonstrate the role of a third significant source of produced water in Marcellus shale gas wells: water vapor condensing out of the gas phase. “Water of condensation” is characterized by negligible salinity and an isotopically light composition (i.e., low δ18O and δ2H). For wells with low daily produced water-to-gas volumes, water of condensation can mask the composition of downhole fluids (which represent an evolving mixture of injection fluid and formation water). End member mixing between downhole fluids and water vapor in equilibrium with these fluids at reservoir temperatures can replicate the observed isotopic and Cl compositions of produced water samples. Results demonstrate that water of condensation, which occurs in surface samples from virtually all natural gas production systems, can significantly influence the composition of produced water in gas wells with low water production rates (a common feature of mature shale gas wells). The impact of water of condensation on produced water composition should be considered when investigating the nature and composition of deep formation waters in low permeability gas reservoirs.